This invention relates to an externally crosslinkable aminopoly(meth)acrylate resin free from epxoy groups for water-dilutable lacquers having an amine number of from 30 to 150 and a hydroxyl number of from 30 to 450. The invention also relates to a process for its production and to the use of the resin as a binder for CED lacquers. In addition, the invention relates to CED baths containing this binder and to their use for coating objects having an electrically conductive surface.
In addition to the externally crosslinkable aminopoly(meth)acrylate resins, these CED baths additionally contain crosslinking agents, such as formaldehyde condensates, blocked polyisocyanates and/or resins containing transesterifiable ester groups and, optionally, hydroxyfunctional resins.
It is known that basic organic binder systems dispersed by salt formation in an aqueous medium may be coagulated on a conductive metallic object immersed in an aqueous bath (ED bath) in an electrical field at the cathode. The corrosion-inhibiting primers normally used for this purpose in practice are aminoepoxy resin systems produced by reaction of epoxy resins based on bisphenol A with various amines and/or aminoalcohols. The deposited film is crosslinked by addition of crosslinking components which do not have to be dilutable with water, but which instead are emulsified with the base resin into the lacquer and at the same time coagulated. The thermal crosslinking of the film is carried out by two basically different methods, for example by addition of blocked isocyanates (DE-A-20 57 799, DE-A-21 31 060, DE-A-22 52 536, DE-A-22 65 195, DE-A-26 34 211) or transesterifiable crosslinking agents (EP-A-0 004 090, EP-A-0 012 463, EP-A-0 040 867, EP-A-B 0 066 859, EP-A-0 082 291, DE-A-31 03 642, DE-A-32 15 891, DE-A-32 24 864, DE-A-32 25 874, DE-A-33 15 469, DE-A-33 22 766). State-of-the-art CED baths are generally used solely for applying primers because the qualities used, based on aminoepoxy resin, yellow on stoving and chalk in long-term weathering. Accordingly, the lacquers cannot be pigmented in such a way that they are suitable as surface lacquers. In addition, in primer/surface coat lacquering, stoving of the surface lacquer, where it is carried out at a higher temperature than is necessary for priming, is sometimes likely to produce cloud-like changes in the color of the surface lacquer through cracking products emanating from the primer, thus spoiling the final appearance of the surface coating. These disadvantages were said to be overcome by using a poly(meth)acrylate resin system containing amino groups.
However, any attempt to use the aminopolyacrylates disclosed in various patents encounters certain disadvantages in regard to the properties of the coatings obtained. According to DE-A-15 46 854, acrylic copolymers containing OH groups are produced using from 3 to 30% by weight of at least one ethylenically unsaturated amino compound, for example the dialkylaminoalkyl(meth)acrylate, dialkylamino(meth)acrylamide or 5- to 6-membered N-vinyl heterocycles and are deposited in combination with formaldehyde condensation resins. In this process, the ED baths are in general highly acidified because the crosslinking reaction with melamine resins is acid-catalyzed. This gives rise to the disadvantage that iron components of tanks, pipes and pumps are attacked and the film ends up with a yellow, often cloud-like discoloration through the iron ions.
According to DE-A-20 57 799, copolymers containing amino groups are produced by copolymerization of amine-containing esters of monofunctional or difunctional unsaturated acids, such as tert.-butylaminoethylmethacrylate or dimethylaminoethylmethacrylate, or vinyl-containing heterocyclic compounds, such as N-vinylpiperidine and, after combination with blocked polyisocyanates, are crosslinked during stoving. Poly(meth)acrylate resins having this chemical structure give rise to difficulties during the electrophoretic deposition of uniform coatings. Even with 100% neutralization, they still show a tendency towards precipitation and give acidic pH values. The blocking agents mentioned are so stable that they are only split off at relatively high temperatures. The products formed in large quantities their removal lead to excessive losses which, as in the case of the phenol used in the Examples, can be harmful to the environment.
According to DE-A-15 95 501, monoethylenically unsaturated monomers containing an epoxy group are copolymerized by emulsion polymerization. These oxirane rings are reacted with amines before, during or after the polymerization. On page 3, middle, it is stated that solution polymerization gives copolymers of, in general, lower molecular weight of which the polymeric particles cannot be stably suspended.
According to DE-A-23 25 177, secondary amines and, with particular advantage, alkanolamines (page 5) because they ensure good dispersibility, are used in a quantity of from 0.1 to 1.0 equivalent per equivalent of epoxy group. If these amines are used in quantities of more than 1.0 equivalent, unreacted amine remains behind and adversely affects the resistance to water and other properties of the hardened coating (page 6).
According to DE-A-28 01 523, epoxy-group-containing copolymers containing from 40 to 90% by weight of unsaturated diene monomers are produced. The epoxy groups are reacted with amine; "in some cases", virtually all the epoxy groups are reacted with amine, although in general excess epoxy groups remain in the resin (bottom of page 4 to top of page 5). Accordingly, the disavantage of epoxy groups remaining in the resin, namely that there is an increase in the viscosity of the resins during storage which can lead to useless products, is accepted in this case, too. Apart from this, these resins tend to yellow after stoving on account of the incorporation of diene monomers.
All the basic copolymers produced in accordance with these patents frequently give rise to stability problems, as reflected in pronounced tendencies towards thickening and gelation, particularly where strongly basic dialkylamines are incorporated, and in precipitation phenomena which appear during ageing of the ED baths or in the surface of the deposited film which presents difficulties because of its irregular thickness distribution or because of its pronounced tendency towards rupture.